Improved method for AFTM G48 corrosion testing of welds (NT TR 548)

Abstract

  The Nordtest project reviews common practices based on ASTM G48 in order to establish and validate an improved method for pre-qualification of stainless steel welds. Different weld qualities of super duplex stainless steel (UNS32750) were tested in the comparative study. The significance of coupon preparation, cut-face pitting and selective corrosion was evaluated by applying additional techniques, such as penetrant examination and propagation test. To provide a better understanding, electrochemical techniques were applied including corrosion potential measurement, cathodic polarisation and determination of the critical pitting temperature (CPT) in accordance with ASTM G150. The study shows that depolarisation may occur during exposure due to pits on cutfaces and/or pickling effects. In case of such events the coupon is exposed at lower potential than intended which may compromise the test result. To avoid this, dippickling is recommended prior to exposure. If weight-loss or presence of cut-face pits indicate depolarisation, it is advised to perform a propagation test or retest of a new coupon. In addition the technique for locating and identifying pits on the exposed coupon has been improved. This involves mandatory dye penetrant examination in combination with microscopy. Based on the obtained results, a new Nordtest method is proposed. The method includes a decision diagram as well as clear acceptance criteria to ensure a high degree of consistency in the test results.